The invention relates generally to control systems for AC induction motors, and more particularly to apparatus and method for the determination and derivation of signals which are representative of the dynamic characteristics of a standard induction motor under load.
The squirrel cage induction motor has structural and dynamic characteristics which make it superior to the DC motor, in particular for variable speed motor drives. These characteristics, however, are also the source of difficult control problems which in the past have prevented the user from obtaining with the AC machine such superior control performance as is inherent in a DC motor drive, despite much research effort and costly development of control techniques.
For instance, the absence of a commutator, or of slip rings, constitutes a definite operational advantage with the induction motor. Nevertheless, by this very fact, the torque producing rotor current is not available for measurement and direct control, which difficulty is not encountered with the armature current of a DC motor. It is also a definite advantage that no separate field winding and field power supply are necessary for the induction motor since three simple conductors suffice to provide both power and excitation. The drawback is that the field level inside the AC machine is not readily available, or controllable, as it would with a separately excited DC machine. The three feeding conductors, from a system point of view, may appear as a rather straightforward mode of excitation. However, when it comes to controlling the induction motor, due to this very simplicity, strong couplings do exist between field producing currents and torque producing currents, and these quantities appear too hopelessly intermingled in the three feeding conductors for any effective manipulation by the control engineer.
The induction motor with its short-circuited rotor is particularly attractive by its ruggedness and related reliability. The squirrel cage motor is also used to best advantage as a fixed speed motor since the demand is extremely large. This is the case for pumps. There, large production volume and low cost require full standardization. It is very important for such applications that control of the motor will not require additional sensors, transducers, or other devices which, while making control design easier, reduce the reliability and deprive the AC induction motor from its basic ruggedness. In particular, modifications internal to the motor should be avoided in order to keep it a standard unit of low cost.
New techniques have been developed which give the AC motor qualities of performance which are comparable to any standard DC motor drive. These techniques, however, up to now have not been implemented without impairing the aforementioned qualities of ruggedness, of reliability, of standard fabrication so much sought for.
One of these new techniques is known as "field orientation". See: Felix Blaschke, "The Principle of Field Orientation as Applied to the New TRANSVEKTOR Closed-Loop Control System for Rotating-Field Machines" in Siemens Review 39 (1972), No. 5, pp. 217-220.
With "Field Orientation" the stator current wave is vectorially resolved in two orthogonal components, one in phase with the internal flux wave and one in quadrature with it. The first component is used to assess and control the excitation level. The second component is used to assess and control the level of generated torque. Through proper vectorial transformation applied to the motor currents and voltages, these two components can be sensed and altered independently, providing a perfectly decoupled system resembling a DC motor, in which separate control of field and armature current is possible. A considerable amount of signal processing is necessary to attain this result and only with the advent of the most recent microcomputer technology has it become reasonable to reduce this concept into practice. See: R. Gabriel, W. Leonhard and C. Nordby, "Field Oriented Control of a Standard AC Motor Using Microprocessors", a paper presented at the 2nd International Conference on Electrical Variable Speed Drives, published by the Institution of Electrical Engineers (1979) pp. 146-150. In this article, a control method is described making use of the dynamic model of an induction motor in order to achieve, with a microprocessor, "field-oriented" control without, like in F. Blaschke, having to insert Hall generators into the motor. However, Gabriel, Leonhard and Nordby fail to achieve control of a standard AC motor because, as explained hereinafter when describing the present invention, they have been unable to ascertain the instantaneous value of the stator resistance when the motor is running. In other words, their dynamic model would require the measurement of temperature on the rotor.
It appears that, with the "field-orientation" control method, precise information is required regarding the phase and magnitude of the rotating flux space vector. As a matter of fact, this is also a requirement with other prior art advanced methods of control. This is the case for instance with the "torque angle" method advocated by L. H. Walker and P. M. Espelage in "A High Performance Controlled-Current Inverter Drive", IEEE Trans. Ind. Appl., Vol. IA-16, pp. 193-202, Mar. Apr. 1980. The information required regarding the flux is in general obtained with sensors embedded in the machine, or it is derived from transducers applied to the shaft.
As shown in U.S. Pat. No. 3,824,437 of F. Blaschke, two voltages indicating the airgap field are derived with two Hall probes displaced at 90.degree. phase relationship. Otherwise, flux regulation has been added with flux rate sensing coils, as in U.S. Pat. No. 4,011,489 of J. P. Franz and A. B. Plunkett. It has also been proposed to derive an indication of the flux directly from the AC voltages at the terminals and of the line currents by a method involving the resistive voltage drop component across the stator. See: U.S. Pat. No. 4,245,181 of A. B. Plunkett. This approach is affected by the thermal drift of the stator resistance, which is the same problem as with the Gabriel, Leonhard and Nordby approach in which the flux is derived from a speed signal by integrating a differential equation relating the rotor current and the magnetizing current. A correction has been proposed which requires that the speed be sensed and known with great accuracy. This necessitates, however, optical incremental transducers to be mounted on the shaft, which are vulnerable in a hostile environment and introduce a weak link in the chain of reliability. More generally, they impair the basic ruggedness of the induction motor and take away the advantage of standardization.
The present invention provides for the use of a standard motor while providing an improved and more advanced approach to the determination of the dynamic characteristics of the induction motor under load thereby to afford the benefit of high performance and reliability with modern control techniques.